Single-mode fiber laser capable of generating multiple dark soliton outputs and method

Abstract

The invention discloses a single-mode fiber laser and method capable of generating multiple dark soliton outputs, and the laser is characterized in that a fiber ring cavity comprises a wavelength division multiplexer, an erbium-doped fiber, a first polarization controller, an analyzer, a second polarization controller, a first output coupler, a fiber isolator, a band-pass filter and an input coupler which are connected in sequence; the input end of the extra-cavity pulse replication unit is connected with the first output coupler through the second output coupler, and the output end of the extra-cavity pulse replication unit is connected with the input coupler. The method comprises the steps that pumping continuous light is coupled into the laser through the wavelength division multiplexer; the erbium-doped fiber is stimulated to radiate a long-wave-band gain pulse; a first polarization controller, an analyzer and a second polarization controller act together to serve as a mode locking starting device, an equivalent saturable absorption effect is generated by nonlinear polarization rotation generated when gain pulses are propagated in an optical fiber, and meanwhile output of a plurality of dark solitons is achieved through the frequency multiplication capacity of an extra-cavity pulse replication unit. The dark soliton generator can output a plurality of dark solitons.

Description

Technical Field

[0001] The present invention belongs to the fields of optical engineering, ultrafast nonlinear fiber optical dynamics, and fiber laser technology, and specifically relates to a single-mode fiber laser and a method for generating multiple dark soliton outputs. Background Art

[0002] A soliton is a wave packet whose parameters remain unchanged while propagating through a medium. The nonlinear Schrödinger equation can be used to describe pulse propagation in optical fiber systems. A dark soliton is a nonlinear wave solution that forms a localized "dip" or phase jump in a uniform background wave and can maintain a stable shape during propagation. Unlike bright solitons, dark solitons appear as troughs with decreasing intensity accompanied by sudden phase changes. Their stability stems from the balance between nonlinear and dispersion effects. Dark solitons have important applications in optical communications, quantum simulation, and nonlinear physics research, and they exhibit unique advantages in noise-resistant transmission and topological defect simulation.

[0003] If the fiber system is energy-conserving, such as pulse propagation in lossless fiber, then solitons can be generated under appropriate conditions. Depending on the sign of the dispersion, dark solitons can be generated in single-mode fibers with normal dispersion. A fiber laser is a dissipative system because of the gain and loss and periodic boundary conditions within the laser cavity. Strictly speaking, the pulses generated in a fiber laser cannot be considered solitons. Kelly et al. demonstrated that even in high-gain fiber lasers, stable pulse evolution can be achieved as long as the soliton period is longer than the energy variation period. Because the stable pulse parameters remain unchanged at any fixed position in the cavity, the stable pulses generated in a fiber laser can be considered solitons. Even in the presence of large periodic energy variations, pulse propagation in a fiber laser can be approximately described by the nonlinear Schrödinger equation.

[0004] Dark solitons have a blackness definition. When the dip is zero, it is defined as a "black soliton"; when the dip is not zero, it is defined as a "gray soliton." Black solitons are generally accompanied by a π phase jump (a sudden phase reversal at the center point), while gray solitons remain in motion and cannot remain stationary. Research has found that there is no threshold for the generation of dark solitons in optical fibers, and any initial dip can evolve into a black soliton; while the generation of black solitons in fiber lasers not only requires an initial pulse, but the presence of gray solitons will also interfere with the generation of new black solitons. Therefore, by specially designing a fiber laser operating in the normal dispersion region to avoid the evolution of gray solitons, it is possible to support the generation of multiple dark solitons. The generation of multiple dark solitons in a single fiber laser will help enhance the understanding of the fiber laser pulse shaping mechanism and deepen the understanding of dark solitons, and has great application and research value. To this end, there is an urgent need to provide a single-mode fiber laser and method that can generate multiple dark soliton outputs. Summary of the Invention

[0005] In response to the problems existing in the above-mentioned prior art, the present invention provides a single-mode fiber laser and method that can generate multiple dark solitons. The laser has a simple structure and low manufacturing cost, and can achieve reliable output of multiple dark solitons. The method has a simple implementation process and low implementation cost, and can achieve stable output of multiple dark solitons.

[0006] To achieve the above-mentioned object, the present invention provides a single-mode fiber laser capable of generating multiple dark soliton outputs, the fiber laser comprising a pump source, a fiber ring cavity, a second output coupler, an extracavity pulse replication unit, and a pass optical switch; the fiber ring cavity comprises a wavelength division multiplexer, an erbium-doped fiber, a first polarization controller, an analyzer, a second polarization controller, a first output coupler, a fiber isolator, a bandpass filter, and an input coupler, which are sequentially arranged along the ring direction;

[0007] The pump port of the wavelength division multiplexer is connected to the pump source through an optical fiber; the erbium-doped optical fiber is a single-mode optical fiber with normal dispersion in the 1550nm band; one end of the erbium-doped optical fiber is connected to the common port of the wavelength division multiplexer; the input end of the first polarization controller is connected to the other end of the erbium-doped optical fiber; the input end of the polarization analyzer is connected to the output end of the first polarization controller; the input end of the second polarization controller is connected to the output end of the polarization analyzer; the input port of the first output coupler is connected to the output end of the second polarization controller; the input end of the optical fiber isolator is connected to the high-energy output port of the output coupler; the operating center wavelength of the bandpass filter is 1550nm, and the 3dB bandwidth range is 7.5nm to 10.5nm; the input end of the bandpass filter is connected to the output end of the optical fiber isolator; the signal input end of the input coupler is connected to the output end of the bandpass filter, and its output end is connected to the signal port of the wavelength division multiplexer;

[0008] The input port of the second output coupler is connected to the low energy output port of the first output coupler;

[0009] The number of the extracavity pulse replication units is one or more. When there are multiple extracavity pulse replication units, the multiple extracavity pulse replication units are cascaded in sequence. The extracavity pulse replication units adopt a Mach-Zehnder interferometer structure, and an optical path delay is introduced in one path so that the output pulse repetition frequency is twice the input pulse repetition frequency. When the multiple extracavity pulse replication units are cascaded, the output pulse repetition frequency is further multiplied. The input end of one or more extracavity pulse replication units is connected to the high-energy output port of the second output coupler.

[0010] The input end of the optical pass switch is connected to the output end of one or more extracavity pulse replication units, and the output end thereof is connected to the input port of the input coupler;

[0011] Among them, the wavelength division multiplexer, the first polarization controller, the analyzer, the second polarization controller, the first output coupler, the second output coupler, the optical fiber isolator, and the pigtail of the bandpass filter are all single-mode optical fibers with normal dispersion in the 1550nm band; the first output coupler and the second output coupler both use optical fiber couplers with an output energy coupling ratio of less than 25%; the first polarization controller, the analyzer, and the second polarization controller work together to make the optical fiber laser operate in the equivalent anti-saturation absorption region through nonlinear polarization rotation, thereby generating dark soliton output, and multiple dark solitons generated by the optical fiber laser are output from the low-energy output port of the second output coupler.

[0012] As a preference, the pump source is a single-mode fiber-coupled semiconductor laser with a central wavelength of 976 nm or 1480 nm, and its output pigtail is a single-mode fiber with normal dispersion in the 1550 nm band, and its output power is greater than 400 mW.

[0013] Preferably, the operating wavelength of the wavelength division multiplexer is 980 / 1550 nm or 1480 / 1550 nm.

[0014] As a preference, the erbium-doped optical fiber has an absorption coefficient of 80 dB / m at 1530 nm and a length of 3 meters.

[0015] As a preference, the first polarization controller is a three-coil rotating polarization controller or a squeeze-type polarization controller; the second polarization controller is a three-coil rotating polarization controller or a squeeze-type polarization controller.

[0016] As a preference, the polarization analyzer is a fiber polarization analyzer.

[0017] As a preference, the optical fiber isolator adopts an isolator with a central wavelength of 1550 nm.

[0018] As a preference, the input coupler is a fiber coupler with an input energy coupling ratio greater than 80%.

[0019] As a preference, the light-through switch is a space-isolating switch.

[0020] In the present invention, the wavelength division multiplexer (WDM) allows the pump light emitted by the pump source to be coupled into the resonant cavity, while also allowing the dark solitons received by the input coupler to be introduced into the fiber ring cavity, thereby stabilizing the energy of the output dark solitons. By installing an erbium-doped fiber on the output side of the common port of the WDM, it absorbs and emits photons and amplifies optical signals within the 1550nm range, effectively compensating for optical signal losses during transmission, extending transmission distance, and improving signal quality. By installing a first polarization controller on the output side of the erbium-doped fiber, the polarization and loss of the optical pulses in the resonant cavity can be adjusted. By installing an analyzer on the output side of the first polarization controller, the polarization orientation of the optical pulses passing through the analyzer can be defined. By installing a second polarization controller on the output side of the analyzer, the polarization and loss of the optical pulses in the resonant cavity can be further adjusted. Furthermore, the first, analyzer, and second polarization controllers, arranged sequentially, act together as a mode-locking initiation device, thereby utilizing the nonlinear polarization rotation generated by the gain pulse propagating within the fiber to produce an equivalent saturable absorption effect, achieving pulse mode locking. By setting up an optical fiber isolator, the laser can be limited to unidirectional operation. In the optical fiber laser of the present invention, erbium-doped optical fiber with normal dispersion and passive optical fiber with normal dispersion are used. At the same time, making the optical fiber laser work in the normal dispersion region and introducing a 7.5nm-10.5nm bandpass filter and setting up an extracavity pulse replication unit with a frequency doubling effect are the keys to the output of dark solitons by the laser of the present invention. Making the optical fiber laser work in the normal dispersion region can make the gain pulse be shaped by the pulse, thereby making the optical fiber laser working in the anti-saturation absorption region generate and output dark solitons. Combined with the increase of pump power and the frequency doubling effect of the extracavity pulse replication unit, 2 or even 2 n The output of a dark soliton.

[0021] The laser has a simple structure and low manufacturing cost, and can achieve reliable output of multiple dark solitons.

[0022] The present invention also provides a method for generating multiple dark soliton outputs, using a single-mode fiber laser that can generate multiple dark soliton outputs, comprising the following steps:

[0023] Step 1: Use a pump source to provide continuous pump light, and couple the continuous pump light into the fiber laser through a wavelength division multiplexer;

[0024] Step 2: Use the erbium-doped fiber to absorb the continuous pump light and stimulate the emission of long-wavelength gain pulses. The generated gain pulses oscillate in the fiber laser cavity and run unidirectionally in a counterclockwise direction.

[0025] Step 3: The first polarization controller, the analyzer, and the second polarization controller work together as a mode-locking initiation device to achieve pulse mode-locking. The nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber is used to make the fiber laser operate in the anti-saturation absorption region. At the same time, since the fiber laser operates in the normal dispersion region, the counterclockwise transmitted gain pulse is filtered by a bandpass filter, and the gain pulse is subjected to pulse shaping, thereby achieving the output of a single dark soliton.

[0026] Step 4: After achieving the generation of a single dark soliton, increase the pump power of the pump source to increase the inter-soliton background by 1 to 2 W; adjust the delay optical path of the extracavity pulse replication unit so that the output pulse repetition frequency is twice the input pulse repetition frequency. At the same time, n extracavity pulse replication units are cascaded to achieve an output pulse repetition frequency of twice the input pulse repetition frequency. n times; turn off the optical switch immediately after the light is turned on, so that the dark soliton output by the extracavity pulse replication unit is input into the fiber ring cavity through the input coupler, generating a stable 2 n A dark soliton is generated and input into the second output coupler through the low energy output port of the first output coupler, and then output through the low energy output port of the second output coupler.

[0027] The present invention provides a method for generating multiple dark solitons. Based on a pulse shaping technique determined by the balance between normal dispersion and nonlinear effects that satisfy the nonlinear Schrödinger equation, the method utilizes nonlinear polarization rotation mode locking to enable the laser to operate in the equivalent anti-saturation absorption region. Simultaneously, an extracavity pulse replication unit multiplies the repetition frequency of the input dark solitons, thereby outputting multiple dark solitons in a fiber laser. A first polarization controller, an analyzer, and a second polarization controller function together as a locking activation mechanism. The method utilizes the nonlinear polarization rotation generated by the gain pulse propagating within the fiber to enable the laser to operate in the anti-saturation absorption region, ensuring reliable dark soliton output. By operating the laser in the normal dispersion region and utilizing nonlinear polarization rotation, the fiber laser can operate in the equivalent anti-saturation absorption region. This region, where the cavity transfer function is in a negative feedback state, has minimal impact on the gain pulse due to wide bandpass filtering. In this case, the pulse shaping experienced by the gain pulse during propagation is determined by the balance between normal dispersion and nonlinear effects that satisfy the nonlinear Schrödinger equation, thereby achieving dark soliton generation. When stable dark solitons are generated, the background of the dark solitons gradually increases when propagating through the gain fiber, while the darkness of the dark solitons (the ratio of the depression intensity of the dark soliton to the background) remains unchanged. The extracavity pulse replication unit makes the output pulse repetition frequency twice the input pulse repetition frequency while replicating the pulse; the introduction of the extracavity pulse replication unit avoids the gray soliton evolution stage in the cavity, thereby achieving the generation of multiple stable dark solitons. The laser of the present invention uses a gain fiber with normal dispersion and a passive fiber with normal dispersion. Allowing the fiber laser to operate in the normal dispersion region and introducing a 7.5nm to 10.5nm bandpass filter is the basis for the laser of the present invention to achieve dark soliton output;

[0028] The method has a simple implementation process and low implementation cost, and can achieve stable output of multiple dark solitons. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the structure of the fiber laser in the present invention;

[0030] Figure 2 A time domain diagram of a single dark soliton output by a numerically simulated laser according to an embodiment of the present invention;

[0031] Figure 3 A spectrum diagram of a single dark soliton output by a numerically simulated laser provided in an embodiment of the present invention;

[0032] Figure 4 A time domain diagram of two dark solitons output by a numerically simulated laser according to an embodiment of the present invention;

[0033] Figure 5 Spectral diagram of two dark solitons output by a numerically simulated laser according to an embodiment of the present invention;

[0034] Figure 6 A time domain diagram of four dark solitons output by a numerically simulated laser according to an embodiment of the present invention;

[0035] Figure 7 This is a spectrum diagram of four dark solitons output by a numerically simulated laser according to an embodiment of the present invention.

[0036] In the figure: 1. Pump source, 2. Wavelength division multiplexer, 2a. Pump port, 2b. Signal port, 2c. Common port, 3. Erbium-doped fiber, 4. First polarization controller, 5. Analyzer, 6. Second polarization controller, 7. First output coupler, 8. Fiber isolator, 9. Bandpass filter, 10. Input coupler, 11. Second output coupler, 12. Extracavity pulse replication unit, 13. Optical switch, 14. Fiber ring cavity. DETAILED DESCRIPTION

[0037] The present invention will be further described below with reference to the accompanying drawings.

[0038] like Figures 1 to 7 As shown, the present invention provides a single-mode fiber laser capable of generating multiple dark soliton outputs, the fiber laser comprising a pump source 1, a fiber ring cavity 14, a second output coupler 11, an extracavity pulse replication unit 12, and a pass optical switch 13; the fiber ring cavity 14 comprises a wavelength division multiplexer 2, an erbium-doped fiber 3, a first polarization controller 4, an analyzer 5, a second polarization controller 6, a first output coupler 7, a fiber isolator 8, a bandpass filter 9, and an input coupler 10, which are sequentially arranged along the ring direction;

[0039] The pump port 2a of the wavelength division multiplexer 2 is connected to the pump source 1 through an optical fiber; the erbium-doped optical fiber 3 is a single-mode optical fiber with normal dispersion in the 1550nm band; one end of the erbium-doped optical fiber 3 is connected to the common port 2c of the wavelength division multiplexer 2; the input end of the first polarization controller 4 is connected to the other end of the erbium-doped optical fiber 3; the input end of the polarization analyzer 5 is connected to the output end of the first polarization controller 4; the input end of the second polarization controller 6 is connected to the output end of the polarization analyzer 5; the input port of the first output coupler 7 is connected to the output end of the second polarization controller 6; the input end of the optical fiber isolator 8 is connected to the high-energy output port of the output coupler 7; the operating center wavelength of the bandpass filter 9 is 1550nm, and the 3dB bandwidth range is 7.5nm~10.5nm; preferably, its pigtail type is Corning MetroCor. The input end of the bandpass filter 9 is connected to the output end of the optical fiber isolator 8; the signal input end of the input coupler 10 is connected to the output end of the bandpass filter 9, and its output end is connected to the signal port 2b of the wavelength division multiplexer 2;

[0040] The input port of the second output coupler 11 is connected to the low energy output port of the first output coupler 7;

[0041] The number of the extracavity pulse replication units 12 is one or more. When there are multiple extracavity pulse replication units 12, the multiple extracavity pulse replication units 12 are cascaded in sequence. The extracavity pulse replication units 12 adopt a Mach-Zehnder interference structure, and introduce an optical path delay in one path so that the output pulse repetition frequency is twice the input pulse repetition frequency. When the multiple extracavity pulse replication units 12 are cascaded, the output pulse repetition frequency is further multiplied. The input end of one or more extracavity pulse replication units 12 is connected to the high-energy output port of the second output coupler 11.

[0042] The input end of the optical pass switch 13 is connected to the output end of one or more extracavity pulse replication units 12, and the output end thereof is connected to the input port of the input coupler 10;

[0043] The pigtails of the wavelength division multiplexer 2, the first polarization controller 4, the analyzer 5, the second polarization controller 6, the first output coupler 7, the second output coupler 11, the optical fiber isolator 8, and the bandpass filter 9 are all single-mode optical fibers with normal dispersion in the 1550nm band; the first output coupler 7 and the second output coupler 11 both use optical fiber couplers with an output energy coupling ratio of less than 25%; the function of the first output coupler 7 is to output multiple dark solitons generated in the cavity, and preferably, its pigtail type is Corning MetroCor. The function of the second output coupler 11 is to output multiple dark solitons generated in the cavity, and preferably, its pigtail type is Corning MetroCor. As a preferred fiber laser, the total length of the optical fiber is 6m.

[0044] The first polarization controller 4, the analyzer 5 and the second polarization controller 6 work together to make the fiber laser operate in the equivalent anti-saturation absorption region through nonlinear polarization rotation, thereby generating dark soliton output. The multiple dark solitons generated by the fiber laser are output from the low-energy output port of the second output coupler 11.

[0045] Preferably, the pump source 1 is a single-mode fiber-coupled semiconductor laser with a center wavelength of 976 nm or 1480 nm. The output fiber is a single-mode fiber with normal dispersion in the 1550 nm band, and the output power is greater than 400 mW. Preferably, the output fiber of the pump source 1 is a Corning MetroCor.

[0046] As a preference, the operating wavelength of the wavelength division multiplexer 2 is 980 / 1550 nm or 1480 / 1550 nm. Preferably, the pigtail type is Corning MetroCor.

[0047] As a preferred embodiment, the absorption coefficient of the erbium-doped fiber 3 at 1530nm is 80dB / m, and the length is 3 meters. Preferably, the erbium-doped fiber 3 is model EDF80, purchased from OFS, which has a high doping concentration, an absorption peak of 80dB / m@1530nm, a strong gain, and a dispersion coefficient of 40.8ps at 1550nm. 2 Of course, as an alternative, other single-mode erbium-doped optical fibers with normal dispersion in the 1550nm band can be selected.

[0048] Preferably, the first polarization controller 4 is a three-coil rotating polarization controller or a squeeze polarization controller. Preferably, the first polarization controller 4 is a squeeze polarization controller, which is used to adjust the polarization and loss of the optical pulse in the resonant cavity. Preferably, the pigtail type is Corning MetroCor.

[0049] Preferably, the second polarization controller 6 is a three-coil rotating polarization controller or a squeeze polarization controller. Preferably, the second polarization controller 6 is a squeeze polarization controller, which is used to adjust the polarization and loss of the optical pulse in the resonant cavity. Preferably, the pigtail type is Corning MetroCor.

[0050] As a preference, the polarization analyzer 5 is a fiber polarization analyzer, which functions to limit the polarization direction of the light pulse passing through the analyzer. Preferably, the pigtail type is Corning MetroCor.

[0051] As a preference, the optical fiber isolator 8 adopts an isolator with a central wavelength of 1550 nm, and preferably, its pigtail type is Corning MetroCor.

[0052] Preferably, the input coupler 10 is a fiber coupler with an input energy coupling ratio greater than 80%, and its function is to introduce dark solitons with doubled repetition frequency generated after being processed by the pulse replication unit 12. Preferably, its pigtail type is Corning MetroCor.

[0053] As a preferred embodiment, the light-passing switch 13 is a space-isolating switch.

[0054] In the present invention, the wavelength division multiplexer (WDM) allows the pump light emitted by the pump source to be coupled into the resonant cavity, while also allowing the dark solitons received by the input coupler to be introduced into the fiber ring cavity, thereby stabilizing the energy of the output dark solitons. By installing an erbium-doped fiber on the output side of the common port of the WDM, it absorbs and emits photons and amplifies optical signals within the 1550nm range, effectively compensating for optical signal losses during transmission, extending transmission distance, and improving signal quality. By installing a first polarization controller on the output side of the erbium-doped fiber, the polarization and loss of the optical pulses in the resonant cavity can be adjusted. By installing an analyzer on the output side of the first polarization controller, the polarization orientation of the optical pulses passing through the analyzer can be defined. By installing a second polarization controller on the output side of the analyzer, the polarization and loss of the optical pulses in the resonant cavity can be further adjusted. Furthermore, the first, analyzer, and second polarization controllers, arranged sequentially, act together as a mode-locking initiation device, thereby utilizing the nonlinear polarization rotation generated by the gain pulse propagating within the fiber to produce an equivalent saturable absorption effect, achieving pulse mode locking. By setting up an optical fiber isolator, the laser can be limited to unidirectional operation. In the optical fiber laser of the present invention, erbium-doped optical fiber with normal dispersion and passive optical fiber with normal dispersion are used. At the same time, making the optical fiber laser work in the normal dispersion region and introducing a 7.5nm-10.5nm bandpass filter and setting up an extracavity pulse replication unit with a frequency doubling effect are the keys to the output of dark solitons by the laser of the present invention. Making the optical fiber laser work in the normal dispersion region can make the gain pulse be shaped by the pulse, thereby making the optical fiber laser working in the anti-saturation absorption region generate and output dark solitons. Combined with the increase of pump power and the frequency doubling effect of the extracavity pulse replication unit, 2 or even 2 n The output of a dark soliton.

[0055] The laser has a simple structure and low manufacturing cost, and can achieve reliable output of multiple dark solitons.

[0056] The present invention also provides a method for generating multiple dark soliton outputs, using a single-mode fiber laser that can generate multiple dark soliton outputs, comprising the following steps:

[0057] Step 1: Use pump source 1 to provide continuous pump light, and couple the continuous pump light into the fiber laser through wavelength division multiplexer 2;

[0058] Step 2: The erbium-doped fiber 3 absorbs the continuous pump light and stimulates the emission of long-wavelength gain pulses. The generated gain pulses oscillate in the fiber laser cavity and run unidirectionally in a counterclockwise direction.

[0059] Step 3: The first polarization controller 4, the analyzer 5, and the second polarization controller 6 act together as a mode-locking starting device to achieve pulse mode-locking. The nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber is used to make the fiber laser operate in the anti-saturation absorption region, that is, the cavity transfer function is in a negative feedback state. At the same time, since the fiber laser operates in the normal dispersion region, the counterclockwise transmitted gain pulse is filtered by the bandpass filter 9, and the gain pulse is subjected to pulse shaping, thereby achieving the output of a single dark soliton.

[0060] Because the laser operates in the normal dispersion region, when the laser operates in the anti-saturation absorption region, the pulse shaping of the gain pulse during propagation can be described by the nonlinear Schrödinger equation, which is mainly determined by the balance between nonlinearity and normal dispersion, so that dark soliton output can be obtained.

[0061] Step 4: After achieving the generation of a single dark soliton, increase the pump power of the pump source 1 so that the inter-soliton background increases by 1 to 2 W, so as to slightly increase the dark soliton background; adjust the delay optical path of the extracavity pulse replication unit 12 so that the output pulse repetition frequency is twice the input pulse repetition frequency, and at the same time, cascade n extracavity pulse replication units 12 to achieve an output pulse repetition frequency of twice the input pulse repetition frequency. n times; turn off the optical switch 13 immediately after the light is turned on, so that the dark soliton output by the extracavity pulse replication unit 12 is input into the fiber ring cavity 14 through the input coupler 10, generating a stable 2 n A dark soliton is generated and input into the second output coupler 11 through the low energy output port of the first output coupler 7 , and then output through the low energy output port of the second output coupler 11 .

[0062] In order to effectively verify the effect of the fiber laser in the present invention in generating dark solitons, numerical simulations were performed. The time domain diagram and spectrum diagram of a single dark soliton output by the laser obtained by numerical simulation are shown in Figure 2. Figure 2 and 3 As shown, the time domain diagram and spectrum diagram of the two dark solitons are respectively as follows Figure 4 and 5 shown. Figure 6 and Figure 6 The time domain diagrams and spectrum diagrams of four dark solitons are given. Figure 2 、 Figure 4 and Figure 6 It can be seen that the depression of the dark soliton goes directly to zero, that is, the blackness of the dark soliton reaches 1.

[0063] The present invention provides a method for generating multiple dark solitons. Based on a pulse shaping technique determined by the balance between normal dispersion and nonlinear effects that satisfy the nonlinear Schrödinger equation, the method utilizes nonlinear polarization rotation mode locking to enable the laser to operate in the equivalent anti-saturation absorption region. Simultaneously, an extracavity pulse replication unit multiplies the repetition frequency of the input dark solitons, thereby outputting multiple dark solitons in a fiber laser. A first polarization controller, an analyzer, and a second polarization controller function together as a locking activation mechanism. The method utilizes the nonlinear polarization rotation generated by the gain pulse propagating within the fiber to enable the laser to operate in the anti-saturation absorption region, ensuring reliable dark soliton output. By operating the laser in the normal dispersion region and utilizing nonlinear polarization rotation, the fiber laser can operate in the equivalent anti-saturation absorption region. This region, where the cavity transfer function is in a negative feedback state, has minimal impact on the gain pulse due to wide bandpass filtering. In this case, the pulse shaping experienced by the gain pulse during propagation is determined by the balance between normal dispersion and nonlinear effects that satisfy the nonlinear Schrödinger equation, thereby achieving dark soliton generation. When stable dark solitons are generated, the background of the dark solitons gradually increases when propagating through the gain fiber, while the darkness of the dark solitons (the ratio of the depression intensity of the dark soliton to the background) remains unchanged. The extracavity pulse replication unit makes the output pulse repetition frequency twice the input pulse repetition frequency while replicating the pulse; the introduction of the extracavity pulse replication unit avoids the gray soliton evolution stage in the cavity, thereby achieving the generation of multiple stable dark solitons. The laser of the present invention uses a gain fiber with normal dispersion and a passive fiber with normal dispersion. Allowing the fiber laser to operate in the normal dispersion region and introducing a 7.5nm to 10.5nm bandpass filter is the basis for the laser of the present invention to achieve dark soliton output;

[0064] The method has a simple implementation process and low implementation cost, and can achieve stable output of multiple dark solitons.

Claims

1. A single-mode fiber laser capable of generating multiple dark soliton outputs, the fiber laser comprising a pump source (1), characterized in that: It also includes a fiber ring cavity (14), a second output coupler (11), an extracavity pulse duplication unit (12), and a light switch (13); the fiber ring cavity (14) includes a wavelength division multiplexer (2), an erbium-doped fiber (3), a first polarization controller (4), an analyzer (5), a second polarization controller (6), a first output coupler (7), a fiber isolator (8), a bandpass filter (9), and an input coupler (10) arranged in sequence along the ring direction; The pump port (2a) of the wavelength division multiplexer (2) is connected to the pump source (1) via an optical fiber; the erbium-doped optical fiber (3) is a single-mode optical fiber with normal dispersion in the 1550nm band; one end of the erbium-doped optical fiber (3) is connected to the common port (2c) of the wavelength division multiplexer (2); the input end of the first polarization controller (4) is connected to the other end of the erbium-doped optical fiber (3); the input end of the polarization analyzer (5) is connected to the output end of the first polarization controller (4); the input end of the second polarization controller (6) is connected to the output end of the polarization analyzer (5); the first output coupling The input port of the optical fiber isolator (7) is connected to the output end of the second polarization controller (6); the input end of the optical fiber isolator (8) is connected to the high energy output port of the output coupler (7); the operating center wavelength of the bandpass filter (9) is 1550nm, and the 3dB bandwidth range is 7.5nm to 10.5nm; the input end of the bandpass filter (9) is connected to the output end of the optical fiber isolator (8); the signal input end of the input coupler (10) is connected to the output end of the bandpass filter (9), and its output end is connected to the signal port (2b) of the wavelength division multiplexer (2); The input port of the second output coupler (11) is connected to the low energy output port of the first output coupler (7); The number of the extracavity pulse replication units (12) is one or more, and when there are more than one, the multiple extracavity pulse replication units (12) are cascaded in sequence; The extracavity pulse replication unit (12) adopts a Mach-Zehnder interference structure, and introduces an optical path delay in one path so that the output pulse repetition frequency is twice the input pulse repetition frequency. When multiple extracavity pulse replication units (12) are cascaded, the output pulse repetition frequency is further multiplied; the input end of one or more extracavity pulse replication units (12) is connected to the high energy output port of the second output coupler (11); The input end of the optical switch (13) is connected to the output end of one or more extracavity pulse replication units (12), and the output end thereof is connected to the input port of the input coupler (10); The pigtails of the wavelength division multiplexer (2), the first polarization controller (4), the analyzer (5), the second polarization controller (6), the first output coupler (7), the second output coupler (11), the optical fiber isolator (8), and the bandpass filter (9) are all single-mode optical fibers with normal dispersion in the 1550nm band; the first output coupler (7) and the second output coupler (11) both use optical fiber couplers with an output energy coupling ratio of less than 25%; the first polarization controller (4), the analyzer (5), and the second polarization controller (6) work together to make the optical fiber laser work in an equivalent anti-saturation absorption region through nonlinear polarization rotation, thereby generating dark soliton output, and multiple dark solitons generated by the optical fiber laser are output from the low-energy output port of the second output coupler (11).

2. The single-mode fiber laser capable of generating multiple dark soliton outputs according to claim 1, characterized in that: The pump source (1) is a single-mode fiber-coupled semiconductor laser, the center wavelength of which is located at 976nm or 1480nm, the output fiber is a single-mode fiber with normal dispersion in the 1550nm band, and the output power is greater than 400mW.

3. The single-mode fiber laser capable of generating multiple dark soliton outputs according to claim 2, characterized in that: The operating wavelength of the wavelength division multiplexer (2) is 980 / 1550 nm or 1480 / 1550 nm.

4. The single-mode fiber laser capable of generating multiple dark soliton outputs according to claim 3, characterized in that: The erbium-doped optical fiber (3) has an absorption coefficient of 80 dB / m at 1530 nm and a length of 3 meters.

5. The single-mode fiber laser capable of generating multiple dark soliton outputs according to claim 1, characterized in that: The first polarization controller (4) is a three-coil rotating polarization controller or a squeeze-type polarization controller; the second polarization controller (6) is a three-coil rotating polarization controller or a squeeze-type polarization controller.

6. The single-mode fiber laser capable of generating multiple dark soliton outputs according to claim 1, characterized in that: The polarization analyzer (5) is a fiber polarization analyzer.

7. The single-mode fiber laser capable of generating multiple dark soliton outputs according to claim 1, characterized in that: The optical fiber isolator (8) adopts an isolator with a central wavelength of 1550nm.

8. The single-mode fiber laser capable of generating multiple dark soliton outputs according to claim 1, characterized in that: The input coupler (10) adopts an optical fiber coupler with an input energy coupling ratio greater than 80%.

9. The single-mode fiber laser capable of generating multiple dark soliton outputs according to claim 1, characterized in that: The light-through switch (13) adopts a space-insulating switch.

10. A method for generating multiple dark soliton outputs, using a single-mode fiber laser capable of generating multiple dark soliton outputs as claimed in any one of claims 1 to 9, characterized in that: The following steps are involved: Step 1: Using a pump source (1) to provide continuous pump light, and coupling the continuous pump light into the fiber laser through a wavelength division multiplexer (2); Step 2: Utilize the erbium-doped fiber (3) to absorb the continuous pump light and stimulate the emission of a long-wavelength gain pulse, the generated gain pulse oscillating in the fiber laser cavity and running unidirectionally in a counterclockwise direction; Step 3: The first polarization controller (4), the analyzer (5) and the second polarization controller (6) are used together as a mode-locking starting device to realize pulse mode-locking, and the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber is used to make the fiber laser work in the anti-saturation absorption region. At the same time, since the fiber laser works in the normal dispersion region, the counterclockwise transmitted gain pulse is filtered by the bandpass filter (9), and the gain pulse is subjected to pulse shaping, thereby realizing the output of a single dark soliton; Step 4: After achieving the generation of a single dark soliton, increase the pump power of the pump source (1) so that the inter-soliton background increases by 1 to 2 W; adjust the delay optical path of the extracavity pulse replication unit (12) so that the output pulse repetition frequency is twice the input pulse repetition frequency; at the same time, n extracavity pulse replication units (12) are cascaded to achieve an output pulse repetition frequency of twice the input pulse repetition frequency. n times; the optical switch (13) is turned off immediately after the light is passed, so that the dark soliton output by the extracavity pulse replication unit (12) is input into the fiber ring cavity (14) through the input coupler (10), generating a stable 2 n A dark soliton is generated and inputted into the second output coupler (11) through the low energy output port of the first output coupler (7), and then outputted through the low energy output port of the second output coupler (11).

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